Temperature control with respect to heater elements constituting a thermal head includes control of a heating time and non-heating time within an application period wherein one printed dot is formed on a printing medium. The heating time refers to a period of time when a main pulse is applied to heat the heater elements to carry out printing, while the non-heating time refers to a period of time when the heated heater elements are cooled.
When the heater elements are heated by application of a main pulse, a portion of the heat is lost at the periphery of the heater elements at the time the printing process starts and at the time isolated printed dots are formed on the printing material during the printing process. This means that the heat generation may become insufficient.
Even if the heater elements are heated by application of a main pulse, if the heater elements adjacent the heated heater elements do not carry out printing, the heat of the heater elements which are heated by the heater elements that do not carry out printing is lost, which means that heat generation may become insufficient.
Also, even if the heater elements are heated by application of a main pulse, if the heater elements have not been heated in the next preceding application period, the temperature of the heater elements at the moment application of the main pulse starts is lower than in the case the heater elements have undergone heating in the next preceding application period. As a result, a rise in the temperature of these heater elements is delayed, which means that heating may become insufficient.
To solve this problem, a sub pulse is applied to compensate for the above-described shortage of heat generation in an application period corresponding to the cases described above. This sub pulse carries out auxiliary heating of the heater elements. The auxiliary heating time obtained by application of a sub pulse follows immediately after the heating time obtained by application of a main pulse.
Furthermore, a sub pulse is applied to compensate for the above-described shortage of heat generation in an application period corresponding to the cases described above. This sub pulse carries out auxiliary heating of the heater elements. The auxiliary heating time obtained by application of a sub pulse follows immediately after the heating time obtained by application of a main pulse.
Accordingly, heating time obtained by application of a main pulse, heating time obtained by application of a sub pulse and non-heating time may all be included in one application period.
Accordingly, in such cases, even if heater elements are heated by application of a main pulse, if the heater elements adjacent the heated heater elements do not carry out printing, a pulse may be applied to the heater elements that do not carry out printing to supply an amount of heat that fails to trigger printing, helping to compensate for the shortage of applied energy.
Furthermore, in some cases, even if heater elements are heated by application of a main pulse, if the heater elements adjacent the heated heater elements do not carry out printing, a pulse may be applied to the heater elements that do not carry out printing to supply an amount of heat that fails to trigger printing, helping to compensate for the shortage of applied energy.
However, as application periods become shorter with higher-speed printing, shorter application periods make it increasingly difficult to adjust the heating times obtained by application of the main pulse and sub pulse as applied in shorter application periods.
As a standard solution, the respective application times for the main pulse and sub pulse can be made shorter correspondingly with shorter application periods. As a result, this offers a solution from the point of view of time. However, in order to heat the heater elements to a point where a shortage of generated heat amount no longer occurs in a shorter heating time, it becomes necessary to increase the applied voltage or otherwise lower the resistance value of the heater elements in the thermal head and increase the current which flows to the heater elements of the thermal head. This requires an improvement in the voltage withstanding property and current capacity with respect to the IC constituting the driving circuit of the thermal head.
Also, another solution that was given includes improving efficiency in transferring heat generated at the heater elements of the thermal head to the printing medium. For this purpose, it is necessary to improve the heat-transfer performance of a thin-film portion in the thermal head comprising heater elements with respect to the printing medium.
However, the above-described solutions exceed the framework of any regular study, which inevitably leads to higher costs.
Accordingly, even in the case the above-described solutions cannot be applied, the application period needs to be shortened in order to increase printing speed, and the ratio of the respective types of heating times using the main pulse or otherwise the sub pulse needs to be increased to secure the necessary heat generation amount required for printing in a shorter application period. As a result, the ratio of the non-heating time will inevitably become shorter. Thus, as the time required for cooling the heater elements which constitute the thermal head and are subject to a temperature increase becomes shorter, successive printing leads to heat accumulation which in turn leads to an uncontrollable rise in the temperature of the heater elements constituting the thermal head. This causes problems from the point of view of printing quality, such as the so-called [print blurring]/[printing tailing].